This invention relates to an echo cancelling circuit for use in a long-distance telephone network and, in particular, in a long-distance conference communication system.
Recently, a long-distance conference communication teleconference system has been developed which is suitable for a conference concurrently held at distant locations, namely, auditoria geographically spaced from one another. Such a system is very effective for saving money, time, and labor of participants attending the conference.
In general, an echo canceller circuit is coupled to a telephone transmitter, a telephone receiver, a sending path, and a receiving path. The circuit receives a send-in signal from the transmitter and sends a send-out signal to a remote party through the sending path. The circuit furthermore receives a receive-in signal from the remote party through the receiving path. The receive-in signal is supplied to the receiver for reproduction of the voice of the remote party. The signal supplied to the receiver will be called a receive-out signal. The circuit is for preventing a part of the receive-in signal from being sent back to the remote party as an echo signal in the send-out signal.
When such an echo cancelling circuit is used in a long-distance conference system, a microphone is used in place of the telephone transmitter. A loudspeaker is used in lieu of the telephone receiver so that the attendants in a particular auditorium can listen to the speech of a speaker in a remote auditorium.
With this system, it should be considered that the loudspeaker is unfavorably acoustically coupled to the microphone. Even a disagreeable howling takes place when the loudspeaker is acoustically closely coupled to the microphone. Such a howling is comparatively readily avoided in a known manner.
In addition to avoiding the howling, it is required in the system to weaken acoustic coupling between the loudspeaker and the microphone. This is because an increasing number of the loudspeakers and the microphones are arranged in the system with an increase of the auditoria and, as a result, acoustic coupling in a total system is apt to become strong.
Furthermore, when each auditorium is spaced far from the others and is, in particular, communicable through a satellite with one another, a speaker's voice sent from one microphone in a near-end auditorium is returned back to the near-end auditorium, as so called an echo electric signal, through acoustic coupling between the loudspeaker and the microphone at a far-end auditorium. Such an echo electric signal is reproduced into an acoustic noise about one second after the speaker's voice is sent.
Acoustic coupling between the loudspeaker and the microphone should be related to reverberation of a phonic sound in each auditorium. Such reverberation induces a reverberation electric signal into a send-in signal sent from the microphone. The reverberation electric signal usually lasts for a considerably long duration as compared with the echo electric signal. If the reverberation electric signal was completely suppressed or cancelled in each auditorium, the echo electric signal would not be returned back from the far-end auditorium to a near-end auditorium.
In a conventional conference communication system, a voice switch has been used to suppress the reverberation electric signal. As will later be described with reference to a few of several figures of the accompanying drawing, the voice switch comprises a first variable attenuator between a microphone and a sending path, a second variable attenuator between a loudspeaker and a receiving path, and a comparator for carrying out comparison between a send-in signal and a receive-in signal. The first and the second attenuators are controlled in accordance with a result of comparison at the comparator. With the voice switch, the reverberation electric signal is favorably removed by adjusting the first and the second attenuators as long as a total attenuation of the voice switch is comparatively small. However, when the total attenuation becomes large, an audio electric signal is widely varied in level. Such a variation brings about a phenomenon like a fading.
Alternatively, it may be possible to utilize, as an echo cancelling circuit for the conference communication system, an echo canceller used in a long-distance telephone network. However, the echo canceller has a serious problem to be solved in order to apply the echo canceller to the conference communication system, as will later be described with reference to a few figures of the accompanying drawing.